Low cost, small packaging, and good reliability are some of the reasons bevel gear differentials have remained a standard for automobiles and trucks since differentials first entered widespread use. Their chief competition arises from specialty products that offer enhanced performance options. One such option provides a traction advantage by resisting normal differential action.
Most automotive differentials provide the primary function of dividing drive power between a pair of drive wheels or between a pair of front and rear axles while allowing the two wheels or axles to rotate at different speeds to accommodate vehicle turning motions, uneven terrain, or differences between the drive wheels themselves. Drive power rotates a differential housing around a common axis of a pair of drive shafts that are connected to the respective drive wheels or axles. Within the housing, a drive train interconnects the drive shafts for equal and opposite directions of rotation (referred to as a -1 speed ratio).
The drive torque that can be applied to the housing is equal to the sum of the torques that can be sustained by the two drive shafts. The torques transmitted to the two drive shafts are normally close to equal, differing only by a small amount of friction generated by the gear train interconnecting them. A traction advantage can be obtained by increasing the friction generated by the gear train or by adding supplemental frictional devices that can support fixed torque differences between the drive shafts.
If the friction opposing relative rotation of the drive shafts is proportional to the drive torque applied to the housing (i.e., a measure of gear train efficiency), then a ratio of torques that can be supported between the drive shafts remains constant over the range of drive torques--a characteristic referred to as "torque proportioning". Relative rotation of the drive shafts (differentiation) occurs when the maximum ratio or "torque bias ratio" is reached. Supplemental frictional devices generally add a fixed amount of friction, referred to as a "preload", which must be overcome regardless of the applied drive torque to permit differentiation.
Many automotive designers prefer options for selecting the frictional characteristics of differentials to assist performance goals. Supplemental frictional components such as springs, clutch plates, and viscous couplings have been added to bevel gear differentials to extend their range of performance. However, if torque proportioning is required, where the friction is a proportional function of the drive torque, other types of gear differentials are normally chosen.
Differential gear trains of parallel-axis (e.g., helical gears) and cross-axis (e.g., worm gears) have been designed to cover a wide range of torque bias ratios. Parallel-axis gear differentials typically support bias ratios through ranges up to around 3 to 1, and cross-axis gear differentials support higher bias ratios of up to 5 to 1 or more. The efficiency range of bevel gear differentials has been much more limited, producing bias ratios in the range of only 1.2 to 1. Gear tooth and bearing mount modifications have been made, but pre-loaded clutch packs or exterior couplings remain the primary way of controlling friction in bevel gear differentials.